1. Field of the Invention
The present invention relates to an electrolysis of an aqueous solution of sodium chloride. More particularly, it relates to an electrolysis of an aqueous solution of sodium chloride at low voltage, at high current efficiency and remains stable for a long time so as to be satisfactory as an industrial process.
2. Description of the Prior Arts
The ion-exchange membrane type electrolysis of sodium chloride which employs an ion-exchange membrane as a diaphragm, has been studied as a process for producing chlorine and sodium hydroxide of high purity which does not substantially contain sodium chloride.
However, an aqueous solution of sodium chloride is contacted through an ion-exchange membrane with an aqueous solution of sodium hydroxide having relatively high concentration in the ion-exchange membrane type electrolysis, whereby calcium ions and magnesium ions in the aqueous solution of sodium chloride, contact with hydroxyl ions in the ion-exchange membrane to produce insoluble hydroxides and to form fine precipitates. Accordingly, it causes serious problems such as the increase of electrolytic voltage and the decrease of current efficiency and the decrease of strength of the ion-exchange membrane and the deterioration of the structure of the membrane during the operation.
In order to overcome these difficulties, it is necessary to purify the aqueous solution of sodium chloride as the raw material fed to an anode compartment, to a higher degree than with the electrolysis in the conventional mercury process or the asbestos diaphragm process.
The aqueous solution of sodium chloride fed in the conventional ion-exchange membrane process has more than about 0.1 to 0.5 mg/liter of calcium concentration as disclosed in Japanese Unexamined Patent Publication Nos. 52698/1973 and No. 86100/1976.
It has been considered to substantially overcome said difficulties by purifying it to said level.
However, the inventors have studied the fluorinated cation exchange membranes having carboxylic acid groups as functional groups which have been known as a high performance membrane in an electrolysis of an alkali metal compound. As a result, the inventors have found that the allowable calcium concentration in an aqueous solution of sodium chloride (brine) for a stable electrolysis for a long time at low voltage and high current efficiency, so as to be satisfactory as an industrial process, should be lower than the proposed calcium concentration of the prior art though it depends upon ion-exchange capacity, and a concentration of an aqueous solution of sodium hydroxide in a cathode compartment.
That is, the inventors have studied the effect of small amounts of calcium component in an aqueous solution of sodium chloride (brine) under conditions employing the fluorinated cation exchange membrane having an ion-exchange capacity of 0.8 to 2.0 meq/g dry polymer and having carboxylic groups as functional groups, and maintaining the concentration of the aqueous solution of sodium hydroxide in the cathode compartment in a range of 20 to 45 wt.% (See Experiment 1). As a result, the following new fact has been found.
In the case of the production of 40% aqueous solution of sodium hydroxide in the cathode compartment by an electrolysis of an aqueous solution of sodium chloride at a calcium concentration of 0.005 to 0.5 mg/liter, the Ca.sup.++ precipitation velocity (b) (.mu.g/hour.multidot.cm.sup.2) in the absorption of Ca.sup.++ into the membrane was measured from the calcium content in the membrane during the electrolysis for definite times.
FIG. 1 shows the relation between the Ca.sup.++ concentration (mg/l) in the aqueous solution of sodium chloride (charged brine) and the Ca.sup.++ precipitation velocity (.mu.g/hour.multidot.cm.sup.2).
As it is clearly understood from the result of FIG. 1, the Ca.sup.++ ion precipitation velocity is zero at the calcium concentration of 0.02 mg/liter on abscissa.
Accordingly, when the calcium concentration in the aqueous solution of sodium chloride (brine) is lower than 0.02 mg/liter, calcium ion is not precipitated into the membrane whereby the accumulation of calcium component is not caused.
The critical calcium concentration in the aqueous solution of sodium chloride (brine) is measured under varying concentrations of the aqueous solution of sodium hydroxide in the cathode compartment. The result is shown in FIG. 2.
In accordance with these findings, the Ca.sup.++ precipitation velocity to the cation exchange membrane is zero and the accumulation of calcium in the membrane is not caused when the calcium concentration in the aqueous solution of sodium chloride (brine) is kept lower than the critical calcium concentration shown in FIG. 2, in the production of sodium hydroxide at the specific concentration.
However, according to further studies by the inventors, it has been found that the deterioration of the electrochemical characteristics of the cation exchange membrane in the electrolysis of an aqueous solution of sodium chloride is not found during the electrolysis of an aqueous solution of sodium chloride containing calcium component at higher than the critical calcium concentration and the accumulation of calcium component is gradually caused at calcium concentration in a range of slightly higher than the critical calcium concentration.
As the result, it has been found that the calcium concentration in the aqueous solution of sodium chloride need not always be maintained lower than the critical calcium concentration but it can be slightly higher than the critical calcium concentration in order to maintain the life of the cation exchange membrane for longer than a half year preferably longer than one year which is required for the ion-exchange membrane employed in an industrial electrolysis.
Thus, the critical value curve becomes gradually flatter upon increasing the concentration of the resulting sodium hydroxide in the range of 20 to 45 wt.% as shown in FIG. 2 and the abovementioned requirements may be satisfied when the calcium concentration is lower than 0.08 mg/liter as the allowable calcium concentration.
It is preferable for longevity of cation exchange membrane to decrease the calcium content in the aqueous solution of sodium chloride (brine) whereby the load of the purification system for removing calcium components is increased. The discovery of the fact that the calcium content in the aqueous solution of sodium chloride can be higher than the critical calcium concentration in the invention is important for industrial operations because the load on the purification system can be mitigated.